This invention relates generally to semiconductors for use in electrical circuits. More particularly, the invention relates to the correction of undesirable surface states in semiconductor devices to realize optimum performance characteristics and to the passivation and stabilization of semiconductor surfaces.
The performance of semiconductor elements in electrical circuits is often adversely affected by characteristics of the surface region of the semiconducting material. Broken or missing bonds and other undesirable conditions may be present primarily as a result of unwanted impurities or an undesirable distribution of necessary impurities that alters the electrical characteristics of the semiconductor material at the surface and, indirectly, deep into the bulk of the material as well. In a p-i-n diode for example, surface channels of n or p type material may extend from the corresponding contact region along the depletion region towards the contact region of opposite type. Adverse effects of such surface channels include excessive leakage current, low break down voltage and excess noise generation. If such a diode is used as a radiation detector, highly uneven response is exhibited in different regions of the radiation receiving volume.
Undesirable surface states may be present as a result of the introduction of impurities during manufacture of the semiconductor element or may arise later from exposure to impurities during storage or use.
Various surface treatments and encapsulation methods have been devised to provide passivation or protection against penetration by impurity atoms without, in the process, adversely affecting the performance characteristics of the material. These are not always wholly effective particularly in the case of certain semiconducting materials which have advantageous electrical properties but which are also extremely prone to develop undesirable surface states.
Germanium, for example, does not have a stable passivating native oxide which can serve as a passivating surface layer. Consequently, germanium devices have been relegated to a relatively small role in the semiconductor industry and are typically used only where the special electrical characteristics of germanium make the lack of stability tolerable and justify special handling and storage procedures. For example, germanium diodes are used for radiation detection in physics research and in medical diagnostic equipment but are found to deteriorate very quickly unless they are maintained at cryogenic temperatures in a very clean vacuum during storage and during handling as well as in use. The expense and inconvenience of maintaining these conditions, particularly during installation of the detectors is considerable. Detection systems involving a large number of such detectors are virtually impossible to fabricate and operate.
Prior efforts to protect germanium diodes from deterioration during storage or use have included the formation of a coating of silicon oxide on the surface. SiO coatings are not desirably effective for several reasons. Such coatings do not reliably compensate for pre-existing adverse surface states but are instead highly sensitive to the initial condition of the surface. In a detector diode, such coatings do not produce flat band conditions at the surface. Instead, response to radiation tends to differ in different regions near the detector surface. Further, SiO coated devices tend to exhibit high leakage current and high inverse frequency noise and the method of application, thermal evaporation, limits such coatings to surfaces of simple geometry.
Essentially similar problems are encountered in the use of certain other semiconductor materials although generally to a lesser extent.
Semiconductor circuit components are commonly formed primarily of certain elements in their crystalline form, the crystalline structure being impregnated with impurities which impart desired electrical properties. It has been recognized that thin films of similar elements, such as silicon or germanium, for example, in the amorphous state may also be used as semiconductor elements in electrical circuits. It has been further recognized that production of such films by sputtering onto an inert substrate in the presence of a controlled atmosphere of hydrogen and argon imparts desirable performance characteristics to the material as described for example by Moustakes, Journal of Electronic Materials, Volume 8, No. 3, pages 391 to 435, 1979, or by Messier et al, Journal of Vacuum Science Technology, Volume 13, No. 5, pages 1060 to 1065, Sept/Oct 1976.
Heretofore such sputtered hydrogenated amorphous semiconducting materials have been viewed essentially as substitutes for circuit components formed of the crystalline material. It has not been recognized that the hydrogenated amorphous form of a semiconducting element might be advantageously utilized in devices basically of the crystalline type to resolve the problems discussed above.